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slacklinejoe
May 28, 2005, 6:19 AM
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Registered: Nov 5, 2003
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Stemming off a few threads in the past I decided I'd go ahead and make a small and limited force calculator for slackline stuff. Eric Matthes did the original formula on his Technical Analysis of Slacklines so I took it a tiny step further and set it up so you can enter in your own numbers on a web page.
The Force Calculator has obvious limitations. It can only calculate in essence the lowest static force that the setup will experience, i.e. standing still. Bounces or movement of any kind will change numbers as will being close to one anchor. None the less, it's just a toy for those technical geeks out there like me. If you want the formula, see his original page. I didn't want to publish any duplicated stuff, just the dynamic calculator page on my site.
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billypilgrim
May 28, 2005, 8:59 PM
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Registered: Sep 18, 2004
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nice little wigget, thanks
There is a previous thread where someone did some testing using a gizmo that measures force. I think it generally lead to the belief that big bouncing or leash falls could double the force of the static setup. If anyone else has done any real dynamic testing I would love to see the numbers. I've been setting up some longer lines this past week and its a bit scary the amount of force you have to apply to get a 90 foot line tight.
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slacklinejoe
May 28, 2005, 9:10 PM
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In reply to: nice little wigget, thanks There is a previous thread where someone did some testing using a gizmo that measures force. I think it generally lead to the belief that big bouncing or leash falls could double the force of the static setup. If anyone else has done any real dynamic testing I would love to see the numbers. I've been setting up some longer lines this past week and its a bit scary the amount of force you have to apply to get a 90 foot line tight.
I didn't think that bouncing doubled the load, leashed falls could though, expecially near the anchor. Hence why highlines go under different safety precautions. I should probably re-read that thread to be sure.
I wish inline dynometers were a lot cheaper, I'd have a lot of use for one in testing.
Regardless of how much pre-tensioning is on the slackline on a 90' the force should be very close to what the calc spits out for standing still in the middle.
If you pre-tension it more, then the sag is reduced raising the forces - let me know if that doesn't make sense. I've found it hard convincing people that what "feels" like rediculously high pre-tension only feels that way at times.
With you standing on it with a 4' drop it should be around 1,000 lbs of tension (weight dependant of course). Even if you managed to double that load in a fall, it stays under half the rate strength of the webbing, of course, considering knots and biners as the weakest link leads to considering precautions.
I think the real benefit of this is that it makes us consider how we design systems to reduce the weakest links in long lines, using sewn loops instead of knots, using stronger knots, upgrading to steel biners or removing metal from the setups, redundant or threaded lines and such.
Of course, I've yet to hear of a failure that wasn't due to A) long term use and abuse on webbing, B) Gates being open, triloading or other biner problems, C) Abrasion or Nylon on Nylon Melt.
Also, to anyone with an inline dynometer, a theory I've been working with involves measuring the increase in line sag during movement to determine the increase in load. I think we could find a correlation between say an increased dip of 6" during a bounce to if the sag was reduced by 6". I think somewhere in there we should be able to avoid the whole unknown elongation % problem that way. It's either that or place a person on a normal scale and measure the increased weight during things like jumping and use that as the input for the slacker's weight to measure force on anchors during a jump.
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